Conventionally, ACLR (Adjacent Channel Leakage Power Ratio) has been measured in W-CDMA system and the like. The adjacent channel leakage power ratio implies a ratio between the power of a center channel and the power of a predetermined adjacent channel. Usually, the adjacent channel leakage power ratio is defined as a ratio between powers within predetermined bands. To obtain a power within the predetermined band, usually, a filter is used to limit the bands, and the power is obtained based on a signal after limiting the bands.
The following section describes how to measure the adjacent channel leakage power ratio in the W-CDMA system. In the W-CDMA system, the adjacent channel leakage power ratio is determined as ten times of a common logarithm (log) of a ratio between the power within a band centered around the center channel frequency and the power within a band separated from the center channel frequency by ±5 MHz or ±10 MHz. As the filter for limiting the bands, an RRC filter (Root Raised Cosine Filter: 3.84 MHz, roll off factor: 0.22) is used. The RRC filter is a filter provides root raised cosine response. FIG. 8 shows how to measure the adjacent channel leakage power ratio according to prior art.
A signal to be measured on which the adjacent channel leakage power ratio is measured is passed through RRC filters 110 which pass signals in predetermined bands centered around a center channel frequency, the center channel frequency ±5 MHz, and the center channel frequency ±10 MHz. Then, the power measuring unit 120 obtains powers P0M, P+5M, P−5M, P+10M, and P−10M in the respective bands based on signals which have passed through the RRC filters. Then, finally, adjacent channel leakage power ratio measuring unit 130 obtains common logarithms (logs) of ratios between the powers of the individual adjacent channels P+5M, P−5M, P+10M, and P−10M and the power of the center channel P0M, and multiplies the logarithms by 10, thereby obtaining the adjacent channel leakage power ratios. Note that when the RRC filter passes a signal in the predetermined band centered around the center channel frequency, the RRC filter sufficiently attenuates a signal outside the predetermined band.
Note that measurement may be conducted while the center frequency of the signal to be measured is moved by ±5 MHz and ±10 MHz to meet the widened band of the signal band. In this case, the band to be measured is lowered (down-converted) to a baseband signal.
However, when the RRC filter is used to pass a signal only in a predetermined band centered around a frequency displaced from the center channel frequency by a certain offset (such as 5 and 10 MHz), the RRC filter does not sufficiently attenuate the signals other than the signal in the predetermined band. Thus, when the powers of the individual adjacent channels P+5M, P−5M, P+10M, and P−10M are obtained, the power outside the bands of the adjacent channels are added to the powers of the adjacent channels, the powers of the adjacent channels are not obtained correctly. In this way, there exists such a problem that the power of the channel is not obtained correctly due to the inferior characteristic of the band-pass filter such as the RRC filter.
Thus, the object of the present invention is to provide a channel power measuring apparatus and the like which can precisely measure the power of a channel such as an adjacent channel.